Glass, rock or steelwool, and a porous ceramic or metallic body are, for example, well known as the sound-absorbing material in a muffler for a machine in general.
FIG. 1(a) shows a conventional muffler for exhaust gas having a cylindrical porous sound-absorbing member 1 composed of, for example, a porour metallic material, a casing 2 which houses the sound-absorbing member 1, and an exhaust gas passage 4 defined within the sound-absorbing member 1. The sound-absorbing member 1 has pores 3.
This muffler, however, has a very short life, since the soot of tarry substance resulting from the composition of gasoline or other fuel, and adhering to the surface and interior of the sound-absorbing member gives rise to its clogging and lowers its sound-absorbing capacity drastically within a relatively short time.
Therefore, the inventor of this invention has made an extensive study of the mechanism which may cause the soot in exhaust gas to adhere to the sound-absorbing member, and found various things as will hereinafter be set forth. As the sound-absorbing member is porous, exhaust gas flows into its interior. The soot in the exhaust gas is composed of fine particles having a diameter of, say, 100 to 1,000 .ANG. which is very small as compared with the diameter of the pores in the porous sound-absorbing member. As the exhaust gas flows into the sound-absorbing member, the soot enters it and adheres to its aggregate. The soot particles become larger by mutual adsorption, and clog the pores in the sound-absorbing material. The adherence of soot is promoted by the presence of, for example, water such as condensed water, or a tarry substance.